1. Technical Field
The present invention relates to an electro-acoustic transducer or a so-called speaker, which is a kind of acoustic equipment and a speaker driver, and more particularly to a speaker suitable for radiation of super low-frequency sound and a driver arrangement therefor.
2. Background Art
Generally, speakers used for acoustic radiation are dynamic speakers (DS) having a construction shown in FIG. 4 of the accompanying drawings. In this type of speaker, a magnet M and a yoke Y are employed for forming a powerful magnetic field across a voice coil VC. When alternating current is supplied to the voice coil VC, a core B connected to the voice coil VC starts vibrating, as indicated by the arrow, with the voice coil VC, thereby radiating a sound wave. The moving part of the dynamic speaker DS is held by a damper D, forming a single oscillation system as a whole.
The dynamic speaker DS generally possesses a frequency-sound pressure characteristic as shown in FIG. 5, as long as the electrical input is fixed. The speaker of this type therefore has been widely used as acoustic equipment. In regard to such characteristic, other types of speakers have a similar tendency.
However, as shown in FIG. 5, the electro-acoustic conversion efficiency suddenly drops below the lowest resonance frequency fo, and therefore the above-described dynamic speaker is unsuitable for use in the radiation of a super low-frequency sound below 50 Hz. As a method of lowering the lowest resonance frequency fo, consideration might be given to increasing the weight of the oscillation system or reducing the strength of the damper. These measures, however, are likely to lower the electro-acoustic conversion efficiency and damping qualities. Furthermore, it is not necessarily totally effective if the electrical input is increased in an attempt to compensate for the lowered efficiency because the exoergic of the voice coil increases.